I don't think it's a bug. At least I hope that it isn't.

I believe..with absolutely zero proof..that when you get a larger timer like that it means that you will get a 'bonus'. Usually that means some extra items. I've had a timer do that with me once when I donated 2 rare tickets and 1 pudding. It increased my timer from 1:30 to 2:30. I think it's something that happens 0.5% of the time or less since Kurito is all about the 'luck of the draw'. However, since this is such a rare occurrence it becomes very hard to test for.